User Create a New Site LIMITATION

Hi

How can I set to ONE user only can create ONE site? Now when user clicking the My Sites on their dashboard, it shows that:

My Sites

Primary Site: jimmy.mysite.com
Create a New Site

How can I disable the above function?

On my Network Setting > Registration Setting, there are few options, currently I set the setting to: Both sites and user accounts can be registered. Is that the correct setting?

Regards

Sylvia